This invention relates to a pressure to voltage converter for use in a gas flow measuring instrument for monitoring the flow of gasses, for example anesthetic gasses administered by an anesthesiologist to a patient undergoing surgery entailing forced ventilation, and more particularly relates to such a converter that is capable of an accurate flow rate indication for any of several gasses that may be used and is capable of such accurate indication when used with any one of a variety of laminar flow sensing elements.
For a patient undergoing surgery the anesthesiologist not only administers an anesthetizing agent to render the patient unconscious but also usually administers a drug having the additional effect of inducing complete muscle paralysis to the extent that there is no involuntary movement of muscles that may interfere with the surgery. For this reason and others it is often necessary to provide a patient with forced pulmonary ventilation. It is especially important with forced ventilation that includes the administration of gasses such a nitrous oxide, oxygen or helium, that are intended to alter the state of the patient, that the flow rate of such body altering drugs be carefully monitored and controlled.
The measurement of the low gas flow rates of interest here, wherein the Reynolds number can conveniently be kept low, may employ a simple laminar flow sensing element (L.F.E.) comprised of a pipe, a bundle of capillaries built into the pipe and two ports, one near each pipe end. The differential pressure existing between the two ports is nearly linearly related to the rate of flow of a gas through the pipe. Thus a pressure to voltage converter connected to the two output pressure ports of the L.F.E. may provide an output voltage very nearly linearly related to the gas flow rate.
To accommodate different applications for several gasses and the lung capacities of small children to adults, the gas flow rates to be measured range from about 0.05 to 800 liters per minute (based upon the use of air). However, as many as six sizes of laminar flow elements are needed to cover this range. It has therefore been necessary to calibrate the combination of each of these L.F.E.'s with the particular pressure to voltage converter that will be used. Such a calibration procedure involves providing the gas of interest at a known flow rate to determine the correlation that will exist between flow rate and output voltage.
Furthermore, any two L.F.E.'s of the same size are likely to have somewhat different sensitivities and each must be calibrated individually in combination with the particular converter that will be used.
In addition, since the different gasses to be used (e.g., air, oxygen and nitrous oxide) have drastically different viscosities, the above mentioned calibrations must be effected for each gas to be used. Thus, for a particular medical procedure, it may be necessary for the anesthesiologist to assemble several combinations of an L.F.E. and a converter for use during the operation and calibrate each combination before they can be used.
It is therefore an object of this invention to provide a differential pressure to voltage converter with which any previously characterized laminar flow element can be readily combined to provide a calibrated output voltage.
It is another object of this invention to provide such a converter whose sensitivity can be simply switched to permit calibrated operation instantly when changing from the use of one gas to another.
It is yet another object of this invention to provide compensation for the small non-linearity that exists in the L.F.E.'s.